I doubt the possibility of a justified expectation of the pleasantness of a surprise. Any situation where information is being intentionally withheld by a human creates a higher prior probability that the surprise is unpleasant for the simple reason that the majority of humans find sharing good news a pleasant act and something they are more likely to do than sharing bad news.

But there are certain contexts where withholding pleasant information is normal, such as a video game where you know your characters are going to get stronger, but don't know the specific abilities they're going to acquire.

If I'm following a story (in nearly any medium) written by someone whose abilities I trust, and there's a persistent mystery in the plot (who was the killer, why does this person have these strange powers, who's this stranger who keeps helping the protagonists and what are their motives, etc.) I can predict that the answer will eventually be revealed, and that I will enjoy learning it. A badly written surprise will be disappointing, but if the story is good, I can predict in advance that the surprises will be enjoyable and usually be right.